This invention relates generally to superconductors and in particular to superconductors made from a solid state reaction between two two-phase alloys.
Intermetallic compounds having an A-15 crystal structure are known to be exceptional superconducting materials. This structure is also referred to as a beta-tungsten crystalline structure. One of the ways in which these compounds are obtained is by a solid state reaction between two alloys in a vacuum or inert atmosphere at an elevated temperature.
The rate of growth of the intermetallic layer is important. The obvious advantage is the economy of reducing the amount of time which metals are maintained at an elevated temperature. Other advantages come from the changes in the physical properties of the intermetallic metal produced by a faster solid state reaction. As the growth rate increases the grain size decreases which improves flux pinning.
The major difficulty associated with manufacturing superconductors with A-15 compounds is fabricating them into usable configurations. First of all the A-15 compounds are extremely brittle and some of the alloys also become brittle through work hardening. Another problem is the adverse effect impurities may have on the completed composite superconductor. Tightness of the bond between the two alloys producing the A-15 compound and grain size of the resulting A-15 compound are also important considerations.
If the solid solution limit of a metal solute in a metal solvent is exceeded, a two phase phenomena is produced in the alloy. The second phase sometimes appears as a precipitation at the grain boundaries of the alloy. In fact, as the concentration of the solute approaches the solid solution limit, discrete particles may begin to faintly form at the grain boundaries.
The second phase precipitation at the grain boundaries can provide crack starters and as a result intergranular fractures occur during processing. On account of the two phase phenomena it was thought that the alloys selected could not exceed the solid solution limit of the solute metal in the solvent metal and that the optimum results would be achieved at a solute concentration of around 2/3 of the solid solution limit.
In patent application Ser. No. 527,000 by Howe, filed Nov. 25, 1974, now U.S. Pat. No. 3,926,684, dated Dec. 16, 1975, it was disclosed that alloys slightly over the solid solution limit of the solute metal could be processed and that the resulting superconductors possessed extraordinary properties. However alloys with a decided two phase structure were not disclosed as useful in fabricating superconductors. It was thought that such alloys would have too much of the A-15 crystal structure to be processable and to provide a high quality product.